1. Technical Field
The present disclosure relates to an imaging device.
2. Description of the Related Art
Digital cameras (digital video cameras or digital still cameras) are widely used in various fields. As is well known, digital cameras use imaging devices, such as charge coupled device (CCD) imaging devices and complementary metal oxide semiconductor (CMOS) imaging devices. These imaging devices have photoelectric conversion elements (also referred to as “photoelectric converter”) typically represented by photodiodes.
Other than imaging devices using photodiodes, imaging devices using photoelectric conversion layers made of, for example, amorphous silicon or an organic material have been also proposed. For example, International Publication No. 2014/083730 discloses a stacked imaging device having a photoelectric conversion layer. In such a stacked imaging device, transistors for reading out electrical signals from individual pixels, such as amplifying transistors and selection transistors, are formed on a semiconductor substrate. The photoelectric conversion layer is disposed on an interlayer insulator formed so as to cover these transistors. The photoelectric conversion layer is electrically connected to the circuitry on the semiconductor substrate by metal lines or metal layers provided in the interlayer insulator. In a stacked imaging device, the photoelectric conversion layer that generates signal charge is located above the semiconductor substrate. Consequently, compared to structures in which various transistors for signal detection and photodiodes are formed on the same semiconductor substrate, stacked structures have the advantage of easily providing a sufficient light receiving area. This makes stacked imaging devices advantageous in achieving higher definition.
This advantage comes with the tradeoff that stacked imaging devices do not allow kTC noise (also called “reset noise”), which is noise introduced by the reset action, to be removed by a simple application of correlated double sampling. This is due to the presence of metal lines or metal layers between the photoelectric conversion layer and the circuitry on the substrate in such a stacked imaging device, making complete signal charge transfer to the floating diffusion difficult. In the imaging device disclosed in International Publication No. 2014/083730, a feedback loop is formed which negatively feeds back the output of a signal readout circuit to the source or drain of a reset transistor in each pixel, and further, a voltage that increases its potential with time is supplied to the gate of the reset transistor. In this way, the imaging device disclosed in International Publication No. 2014/083730 cancels kTC noise.